Hi solhaga,
I had the same problem, I used vinyl glue to seal the back panel and this leaved some leakage. I tried to seal them pouring some glue from the horn mouth and then rotating the cabinet to let it flow along the corners formed by the plywood and the foamcore board.

In my experience vinyl glue is too liquid, it's easily absorbed by wood and "shrink" too much when drying. Next build I'll try a more dense glue.

Solhaga,
What are the channels made of? Black foam core or is that like vinyl? There are no points for internal beauty as long s it sounds good if channels are foam core and cut cleanly, they seal nicely with cheap white glue. They are also stiff in compression and would not require bracing at throat.

Solhaga,
What are the channels made of? Black foam core or is that like vinyl? There are no points for internal beauty as long s it sounds good if channels are foam core and cut cleanly, they seal nicely with cheap white glue. They are also stiff in compression and would not require bracing at throat.